1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device with a gallium-nitride-based semiconductor structure.
2. Description of the Background Art
GaN-, AlGaN-, and InGaN-based semiconductor devices and other wide bandgap semiconductor devices have difficulty in achieving a low-resistance p-type semiconductor layer as compared to achieving an n-type semiconductor layer. It is also difficult for a p-type semiconductor layer to provide low contact resistance for electrode contact with as compared with an n-type semiconductor layer.
Besides, a metal electrode layer formed on a p-type semiconductor layer is often considered insufficient as a layered structure for long-term stable operation. For this reason, Japanese Patent Application Laid-open No. 10-209493, for example, discloses a lamination technique that provides good ohmic properties through the use of palladium (Pd) as a metal electrode layer and through heat treatment in a gas atmosphere containing oxygen. Furthermore, Japanese Patent Application Laid-open No. 11-54798, for example, discloses a technique for reducing a voltage drop through the intervention of a thickness-controlled surface-oxidized film between a GaN semiconductor layer and a metal electrode layer which form a semiconductor electrode layer.
A method of manufacturing such a GaN-based wide bandgap semiconductor device requires either heat treatment in a gas atmosphere containing oxygen at the time of the formation of a metal electrode layer, or the formation of an oxide film on the surface of a p-type GaN semiconductor layer which is to be a semiconductor electrode layer.
If a heat treatment process in a gas atmosphere containing oxygen is necessary in forming a metal electrode layer, metal species that can be used as high-melting-point metals, which are combined with Pd into a layered structure in order to ensure long-term stable operation at current densities as high as or more than 20 kA/cm2, are limited to those metals that are unaffected by oxidation caused by heat treatment in an oxygen atmosphere. This leads to a drawback that a layer-structure metal electrode layer cannot satisfactorily provide long-term stable operation.
The formation of an oxide film on the surface of a p-type GaN semiconductor layer, which is to be a semiconductor electrode layer, is implemented through the process of first removing an initial oxide film on the surface of a p-type semiconductor layer in an HF solution and then controlling the resultant oxide film to a sufficient level by immersion in hot water or oxidation in an oxygen atmosphere. Thus in an actual semiconductor laser element, a structure including an insulation film that is formed to provide a ridge structure for electrical and optical confinement has the problem that the insulation film may be removed, or reduced in thickness, during the process using a HF solution. This will change electrical and optical confinement conditions in a semiconductor laser, thus giving an adverse effect on the laser properties. In other words, there is a problem of degradation in the performance of a GaN-based semiconductor device.